Boundary layers are a manifestation of friction loss. Friction loss is lower in a laminar boundary layer than in a turbulent one. Both boundary layers exist in a typical wing. A fluid passing over a wing beginning at the leading edge flows for a distance with a laminar boundary layer then becomes turbulent close to the leading edge. The frictional losses occur because a no-slip condition at the wing surface imposes a shear in the flow from the surface up to the freestream.
Flow in a laminar boundary layer is smooth and in layers parallel to the solid surface, the layers being called stream surfaces. The friction at the surface between the fluid and the surface results in no velocity there; the velocity of the fluid progressively increases away from the surface to freestream where the effects of the wing surface no longer influence the flow.
Turbulent boundary layer flow is characterized by random interaction of vortex filaments which produce a fuller mean velocity profile than a laminar velocity profile. The fuller profile results from the cross motion of individual fluid particles with respect to the average direction of fluid motion in directions through the boundary layer and normal to the wing surface and transverse to the average direction of fluid motion and parallel to the wing surface. The crossflow exchanges momentum between particles and tends to blunt the velocity profile. The exchange of momentum in turbulent flow produces relatively large shear forces. In a turbulent boundary layer close to the solid surface the crossflow normal to the solid surface must go to zero because of the surface, and at the surface-fluid interface the velocity parallel to the surface must be zero because of friction; the region where this happens is called the laminar sublayer.
Because the random particle motion and exchange of momentum between faster and slower particles in a turbulent boundary layer do not occur in laminar boundary layer flow, the shear stress in a turbulent boundary layer is greater than in the laminar one. As a result, friction loss is greater in a turbulent boundary layer than in a laminar boundary layer.
Another characteristic of turbulent flow is the dominance of inertial forces over viscous forces when compared to laminar flow. In laminar flow disturbances smooth out and disappear because of viscosity; in turbulent flow disturbances are not smoothed out but increase with time, and that is why turbulent flow occurs. Reynolds' number is a measure of the ratio of inertial forces to viscous force. The transition from laminar to turbulent flow occurs usually at a critical Reynolds' number that depends on disturbances in the system being observed. In a majority of environments, turbulent flow occurs at a Reynolds' number of between 1,000 and 3,000. In some environments laminarity has been maintained at much higher Reynolds' numbers.
Reynolds' number includes a dimensional term and a velocity term. In aerodynamic analysis of wings, the dimensional term is usually the distance from the leading edge of the wing and the velocity term is the freestream velocity. In conventional analysis, this convention is perfectly acceptable because the change from laminar to turbulent flow generally depends on the distance from the leading edge and freestream velocity.
Frictional drag in a laminar boundary layer is an order of magnitude smaller than in a turbulent boundary layer. Because the typical wing operates with a turbulent boundary layer there is a corresponding large energy consumption resulting in major penalties on the efficiency and performance of vehicle propulsion systems.
It is well known that the evolution of laminar boundary layer flow into a turbulent boundary layer flow can be prevented by imposing a favorable pressure gradient in the flow: pressure decreasing in the direction of flow. Such a gradient can be provided by body shaping or suction. In this instance, individual fluid particles accelerate in the freestream direction in response to the negative pressure gradient to velocities much greater than crossflow velocities characteristic of turbulence; the velocity in the freestream direction dominates the flow, thus avoiding evolving into turbulence.
Attainment of such a gradient by shaping, however, is limited to the forepart of the body where diverging surfaces exist; the aft part of the body has converging surfaces in the direction of flow that result in adverse, positive pressure gradients in that direction which almost universally result in turbulent flow in practical vehicle application.
The provision of a favorable pressure gradient by suction, on the other hand, can in principle maintain laminar flow throughout the entire length of the body. This mechanism removes particles that have lost velocity in the freestream direction and replaces them with particles having appreciable velocities in the freestream direction, thus maintaining a favorable dominance of freestream velocity over crossflow terms and avoiding a turbulent opportunity. In the past, attempts have been made to provide such gradients through the use of internal suction acting through apertures in the body surface connected to a mechanically operated suction pump. Irregularities in the surface imposed by such apertures, the tendency of the apertures to clog up with foreign material, the network of tubing and plumbing with attendant frictional losses, and the necessity of a mechanical suction pump all burden the internal suction systems and make them impractical.
The payoff for an operable laminar flow system is large, and it is clear that this desirable result can be achieved if a practical means to provide an economic suction source can be devised. A passive system involving no moving parts, involving no external power sources, confined entirely to the external surface of a wing or body, and using only waste energy available for the task would be highly desirable.